The Short Version

A gas furnace burns natural gas (or propane) to produce heat, then a blower fan pushes that heat through your ductwork and into every room. The whole cycle takes about 10–15 minutes from the time your thermostat calls for heat to the time warm air reaches your vents.

Here's what happens step by step.

Step 1 — The Thermostat Calls for Heat

When your home drops below the set temperature, the thermostat sends a 24-volt signal to the furnace control board. This is the starting gun for the entire heating sequence.

Step 2 — The Inducer Motor Starts

Before any gas flows, the inducer (draft) motor spins up to pull fresh air into the combustion chamber and push exhaust gases out through the flue pipe. This pre-purge clears any residual gas and confirms the venting path is clear.

A pressure switch monitors the inducer. If it doesn't see the right pressure — meaning the flue may be blocked or the motor is failing — the furnace shuts down and locks out. This is a safety feature, not a malfunction.

Step 3 — The Igniter Glows

Modern furnaces use a hot surface igniter (HSI) — a small silicon carbide or silicon nitride element that heats to roughly 1,800°F in about 30 seconds. Older furnaces used a standing pilot light, which burned gas continuously. HSI ignition is far more efficient.

Step 4 — The Gas Valve Opens

Once the igniter reaches operating temperature, the control board opens the gas valve. Gas flows to the burners and ignites. A flame sensor — a thin metal rod in the burner flame — confirms ignition within a few seconds. If the sensor doesn't detect a flame, the valve closes and the furnace tries again. After a set number of failed attempts, it locks out and flashes an error code.

Step 5 — The Heat Exchanger Warms Up

Combustion gases flow through the heat exchanger — a series of metal tubes or cells. Return air from your home passes over the outside of these tubes. The heat transfers through the metal walls without the combustion gases ever mixing with your indoor air.

This separation is critical. A cracked heat exchanger allows carbon monoxide to enter your living space. It's the main reason annual furnace inspections matter.

Step 6 — The Blower Fan Turns On

Once the heat exchanger reaches a safe temperature (typically 120–140°F), the blower fan starts. It pulls cool return air from your home, pushes it across the heat exchanger, and delivers warm air through your supply ducts.

The blower doesn't start immediately — if it did, you'd feel a blast of cold air at first. The delay is intentional.

Step 7 — The Cycle Ends

When the thermostat is satisfied, it cuts the signal to the furnace. The gas valve closes and the burners go out. The blower continues running for a minute or two to extract remaining heat from the heat exchanger — this is normal. Then everything shuts down until the next call for heat.

Single-Stage vs. Two-Stage vs. Modulating

Single-stage: The burner is either fully on or fully off. Simple and reliable, but less efficient and can cause temperature swings.

Two-stage: The burner runs at a lower capacity (typically 65%) most of the time and only fires at full capacity on the coldest days. More efficient, quieter, and more even heat distribution.

Modulating: The burner adjusts output continuously — anywhere from 40% to 100% — to match exactly what the home needs. The most efficient option, with the most consistent comfort. Lennox SLP99V and Carrier Infinity 98 are examples.

What AFUE Means

AFUE (Annual Fuel Utilization Efficiency) measures how much of the gas burned actually becomes heat in your home. A 96% AFUE furnace converts 96 cents of every dollar of gas into heat. The other 4 cents goes up the flue as exhaust.

80% AFUE furnaces vent hot exhaust gases through a metal flue pipe. 90%+ furnaces (condensing furnaces) extract so much heat that the exhaust cools to the point of condensing into water — which drains away — and vents through PVC pipe instead of metal.

Key Components at a Glance

• Control board — the brain; sequences every step and stores error codes
• Inducer motor — pulls combustion air in, pushes exhaust out
• Pressure switch — confirms the inducer is working before allowing gas flow
• Hot surface igniter — lights the gas
• Gas valve — controls gas flow to the burners
• Flame sensor — confirms the burners are lit
• Heat exchanger — transfers heat to your air without mixing combustion gases
• Blower motor — moves air through your ductwork
• Limit switch — shuts the furnace off if it overheats
• Filter — protects the blower and heat exchanger from dust buildup

What Goes Wrong (and Why)

No heat: Most often a dirty filter restricting airflow, a tripped limit switch, a failed igniter, or a faulty flame sensor. See our Common Furnace Problems guide for a full breakdown.

Short cycling: The furnace fires and shuts off repeatedly without completing a full heating cycle. Usually caused by an overheating condition — often a dirty filter or blocked return air.

Cracked heat exchanger: The most serious issue. Allows carbon monoxide into your home. Symptoms include a yellow or flickering burner flame, soot around the furnace, or a CO detector alarm. Requires immediate professional inspection.

Chicago-Specific Notes

Chicago winters regularly push furnaces to run 10–14 hours a day during cold snaps. That's a lot of cycles. A furnace that's borderline in October may fail in January when you need it most. Annual tune-ups before heating season — September or October — catch problems before they become emergencies.

Chicago's older housing stock (two-flats, greystones, bungalows) often has original ductwork that restricts airflow. If your furnace short-cycles or rooms heat unevenly, duct condition is worth checking alongside the furnace itself.